Homeostasis

• Homeostasis is a term that is used to refer to the maintenance of a stable equilibrium inside an organism.

• This equilibrium however is not static, rather it is a dynamic one.

• Thus the organism and its internal environment will remain in a balance - a dynamic equilibrium.

• Levels will change but they will fluctuate around the desired levels.

Homeostasis is responsible for the maintenance of:

• Concentration of oxygen and carbon dioxide

• pH of the internal environment

• Concentration of nutrients and waste products

• Concentration of salt and other electrolytes

• Volume and pressure of extracellular fluid

Control Systems

• Depending on the type of organism, the system may be open or closed.

• An open system has no feedback. It may be compared to a light switch.

• The light does not influence whether or not the switch is being turned on or off.

• A closed system uses a feedback mechanism and has two components: a sensor which senses a change and an effector which responds to the change.

• This type of system acts the same way as a thermostat (sensor) and a furnace (effector).

• When the thermostat is adjusted to an appropriate setting the furnace cuts in.

• When the desired temperature is reached, the furnace cuts out.

• Systems in the body use this type of feedback in homeostasis.

Extrinsic vs Intrinsic

• Homeostatic systems can be extrinsic; that is they are controlled externally.

• In higher order organisms like animals, the endocrine and the nervous system are the major feedback controls.

• Homeostatic systems can also be intrinsic; that is only one organ or tissue is controlled locally.

• For example, when muscles are depleted of oxygen and produce more carbon dioxide, there is a dilation of blood vessels that allow more blood, and thus more oxygen into the area. Eventually, the vessels constrict to normal size.

Feedback Systems in Homeostasis

• There are two types of feedback control mechanisms involved in homeostasis. Positive and Negative.

• Negative feedback control mechanisms is used by most body systems. It results in a reverse response.

• TSH production is an example of a negative feedback. As blood levels of TSH rise, the production of TSH is slowed and vice versa. Therefore, blood levels of TSH serve as a feedback for the production of TSH.

• The second type of feedback mechanism is the positive feedback control mechanism.

• As input increases, the response is accelerated. For example, oxytocin is a hormone that is produced during labor as the uterus contracts.

• As levels of oxytocin increases, uterine contractions become stronger.

• As uterine contractions heighten, it signals more oxytocin to be produced.

Body Systems

There are a number of systems in the human body. Our study will be limited to”

– Circulatory– Digestive– Respiratory– Excretory– Immune